Seismic data from both explosive and earthquake sources have been used to constrain the crustal and upper mantle velocity structure beneath the Taupo Volcanic Zone (TVZ), a back-arc basin in North Island, New Zealand. A thin (16 km) quartzo-feldspathic crust is underlain to ~30 km depth by material with P-wave velocities 6.9--7.3 km/s, interpreted as heavily intruded or underplated lower crust. Low seismic velocities of 7.4--7.8 km/s are present in the underlying mantle wedge to at least 100 km depth, which we attribute to the presence of volatiles from dehydration of the subducting Pacific plate. A region of high (0.34) Poisson's ratio is found in the lower crust beneath the TVZ, consistent with the presence of partial melt. This melt fractionates and assimilates crustal material before migrating into the upper crust, where it provides a source for the voluminous rhyolitic magmas of the TVZ.